This application is a 371 application of PCT/JP00/06880 filed Oct. 3, 2000.
This invention relates to optical resin materials, more particularly to non-birefringent optical resin materials that show substantially no birefringence or which show small enough birefringence to cause no adverse effects in practice, as well as applications of such non-birefringent optical resin materials.
Recently, high molecular weight resins are increasingly supplanting the conventional glass-based materials for use not only in general optical parts such as eyeglass lenses and transparent sheets but also in optoelectronic optical parts such as those to be used in laser-related equipment as exemplified by optical disk apparatus for recording acoustic, video, character and other information. This is because optical materials made of high molecular weight resins, namely, optical resin materials, are generally better suited to efficient processing and high-volume production than glass-based optical materials since they are lighter in weight, have higher impact resistance and allow for easier application of molding techniques such as injection molding and extrusion molding.
These characteristics are of course useful for the various kinds of optical parts mentioned above; in addition, they are even more useful when optical resin materials are used in various members that compose the liquid-crystal device as the principal component of a liquid-crystal display. Liquid-crystal displays have come to be used extensively as the display element of various electronic equipment. As their use has expanded, liquid-crystal displays are increasingly required to be lighter and thinner, with the added need for improvements in strength performance such as higher impact resistance. These requirements can be met by effective utilization of those characteristics possessed by high molecular weight resin materials.
Thus, optical resin materials have the potential to show excellent characteristics as optical parts and they are expected to find extensive use in optical parts. In fact, however, they have not been used as much as expected. This is primarily because the products manufactured by applying the aforementioned molding techniques to optical resin materials show by no means small birefringence which sometimes impairs the functions of optical parts in which they are used.
The occurrence of birefringence in high molecular weight resin materials is widely known per se, inclusive of its cause. To be more specific, for almost all high molecular weight resin materials that are commonly used as optical materials, the monomers of which they are formed have optical anisotropy in refractive index and if the polymer is oriented or shows alignment in a given direction, this optical anisotropy of the monomers is expressed to develop birefringence in the high molecular weight resin material. More specifically, the following phenomena take place.
The polymer as produced by polymerization reaction has the linked chains intertwined randomly, i.e., the linked chains of the polymer are not oriented. In this state, the optical anisotropy of one monomer unit cancels that of another monomer unit and the polymer shows no birefringence. However, upon molding such as injection molding or extrusion molding, an external force is applied and the random linked chains in the polymer become oriented, causing the polymer to show birefringence. This state is shown schematically in FIG. 1. As shown, the high molecular weight resin material that has undergone the molding process accompanied by an external force is in such a state that a number of units (monomers) 1 which form the linked chains in the polymer are linked spatially in a specified direction of orientation. And as already mentioned, for almost all high molecular weight resin materials that are commonly used as optical materials, every unit 1 has optical anisotropy in refractive index. In other words, the refractive index npr for a polarized wave component travelling in a direction parallel to that of orientation is different from the refractive index nvt for a polarized wave component in a direction vertical to that of orientation.
As is well known, this optical anisotropy can be expressed by the index ellipsoid. Referring to FIG. 1, each linked unit 1 has an elliptic mark 2 which represents the index ellipsoid. Take, for example, polymethyl methacrylate (PMMA); the refractive index of each unit (methyl methacrylate) 1 is relatively small in the direction of orientation and relatively large in a direction vertical to it. Therefore, the index ellipsoid as viewed at a macroscopic scale is oblong in the vertical direction as indicated by 3 in FIG. 1. In other words, npr is smaller than nvt with polymethyl methacrylate. The difference obtained by subtracting nvt from npr (xcex94n=nprxe2x88x92nvt) is called the value of birefringence in orientation. The following Table 1 lists the values of intrinsic birefringence for typical optical resin materials.
The value of intrinsic birefringence is the value of birefringence exhibited by any one of these polymers when they are fully oriented in a certain direction. In fact, depending on the degree of its orientation, the polymer assumes a value between zero and the intrinsic birefringence.
For example, the polymethyl methacrylate shown in FIG. 1 has xcex94n between xe2x88x920.043 and 0, and polystyrene has xcex94n between xe2x88x920.100 and 0. With polyethylene, xcex94n has a positive value between 0 and +0.044. Hereinafter, if the sign of xcex94n is positive (xcex94n greater than 0), the expression xe2x80x9cthe sign of birefringence is positivexe2x80x9d is to be used and if the sign of xcex94n is negative (xcex94n less than 0), the expression xe2x80x9cthe sign of birefringence is negativexe2x80x9d is to be used.
The birefringence in orientation is particularly problematic in applications where polarization characteristics are important. A typical example of such applications is a group of optical parts in the recently developed write/erasable magnetooptical disk. The write/erasable magnetooptical disk uses polarized beams as reading or writing beams, so if birefringent optical elements (e.g. the disk per se or lenses) are within the optical path, the precision in reading or writing is adversely affected.
An application where the birefringence in the members used plays a more important role is a liquid-crystal device. As is well known, the liquid-crystal device consists of a liquid-crystal layer sandwiched between a polarizer and an analyzer that form crossed or parallel Nicols and the liquid-crystal layer switches between the transmission and non-transmission of light by rotating the plane of polarization of polarized light. Hence, the birefringence of the members which compose the liquid-crystal device poses a great problem which prevents extensive application of optical resin materials to the liquid-crystal device.
With a view to eliminating this problem of birefringence in orientation, various proposals have heretofore been made. A typical example is disclosed in commonly assigned PCT/JP95/01635 (International Publication WO96/06370). According to this technique, a matrix made of a transparent high molecular weight resin is mixed with a low molecular weight organic substance that can be oriented in the same direction as the linked chains in the matrix forming high molecular weight resin are oriented under an external force and the birefringence in orientation of the high molecular weight resin is cancelled out by the birefringence of the low molecular weight organic substance to produce a non-birefringent optical resin material.
To be more specific, the matrix forming high molecular weight resin has birefringence in orientation whose sign is positive or negative and the low molecular weight organic substance added to it exhibits birefringence of opposite sign, whereby the two values of birefringence cancel each other to provide high degree of non-birefringence. If this non-birefringent optical resin material is subject to stress or other external action as during molding, the linked chains in the high molecular weight resin are oriented and the added low molecular weight organic substance is accordingly oriented. The major axes of the index ellipsoids in the oriented low molecular weight organic substance cross the major axes of the index ellipsoids in the high molecular weight resin at right angles and the birefringence in orientation of the overall system is either substantially eliminated or reduced to such a level that it can be regarded as having no birefringence.
This technique has many advantages. For example, the birefringence of the high molecular weight resin can be reduced to almost zero by simply adjusting the loading of the low molecular weight organic substance in accordance with the types of the two components. The low molecular weight organic substance is not substantially involved in the polymerization reaction of the matrix forming high molecular weight resin (i.e., it has no reactivity with the monomer from which the high molecular weight resin is formed) and, hence, there are few limitations on the combination of the high molecular weight resin and the low molecular weight organic substance. In other words, there is high degree of freedom in the choice of the high molecular weight resin. The low molecular weight organic substance generally has greater optical anisotropy in refractive index than the individual unit molecules in the high molecular weight resin; hence, it need be added in comparatively small amounts and the characteristics of the matrix forming high molecular weight resin can be utilized more effectively in the optical resin material. There is no need for design consideration that can prevent the polymer from being oriented and molding technologies such as injection molding and extrusion molding that feature high processing rate and productivity can be freely applied to process the polymer.
Even this salient technique has one major drawback that cannot be neglected in certain applications of the produced optical resin material. The problem arises from the use of the low molecular weight organic compound as the substance to cancel out the birefringence in orientation of the polymer and it is a drop in heat resistance. The low molecular weight organic compound to be added always has varying degrees of plasticizing effect on the matrix forming high molecular weight resin and its glass transition temperature is lowered accordingly. The plasticizing effect of the low molecular weight substance is an advantage to materials that need be given softness by the plasticizer but it is a serious drawback in applications where a certain minimum level of heat resistance is required.
The present invention has been accomplished under these circumstances and has as an object providing a technology that can solve the aforementioned problem of heat resistance in the prior art. More specifically, the invention aims at providing a technology that retains the various advantages of the prior art and which yet can solve the problem of decreased heat resistance.
To attain these objects, the present invention provides, according to one aspect, an optical resin material comprising a transparent high molecular weight resin and a fine inorganic substance which, as the linked chains in the high molecular weight resin are oriented under an external force, is oriented in the same direction as the linked chains and which has birefringence, the birefringence of said inorganic substance cancelling out the birefringence in orientation of the oriented high molecular weight resin.
The term xe2x80x9ccancelling out the birefringence in orientationxe2x80x9d as used herein means bringing the birefringence in orientation close enough to zero. To be more specific, if the high molecular weight resin of interest has a positive birefringence in orientation, it is xe2x80x9ccancelled outxe2x80x9d by being decreased toward zero; if the resin has a negative birefringence in orientation, it is xe2x80x9ccancelled outxe2x80x9d by being increased toward zero. The term also means reducing the absolute value of the birefringence in orientation. The birefringence in orientation need not necessarily be brought to zero but may be brought close enough to zero that there will be no adverse effects in practice.
The present invention is based on the idea disclosed in International Publication WO96/06370 but differs from the prior art in that it is characterized by adding an inorganic substance. The inorganic substance used in the invention is substantially free from the plasticizing effect due to the bearing effect which inevitably occurs in low molecular weight substances or low molecular weight organic compounds. It hence has the advantage of not impairing the intrinsic heat resistance of the high molecular weight resin.
Minerals are typical examples of the inorganic substance and as is observed in their crystals, the inorganic substance in many cases has much greater birefringence than organic compounds. Within the crystals, the constituent atoms have an orderly steric arrangement and in birefringent crystals the arrangement is anisotropic. In polymers, birefringence does not occur before they are drawn or otherwise oriented but this is not the case with crystals and they show birefringence on account of their inherent crystal structures. When light travels through a birefringent crystal, it branches in two waves having orthogonally crossed planes of polarization and there is a direction in which the crystal has the same refractive index for the two waves. The axis parallel to this direction is commonly called the optical axis. An index ellipsoid can be defined for the crystal with this optical axis taken as one of the principal axes. The difference between the refractive indices along the major and minor axes of the index ellipsoid gives the value of birefringence in the birefringent crystal. To give a few examples, calcite (CaCO3) has a birefringent value of xe2x88x920.17, rutile (TiO2) +0.287, magnesite (MgCO3) xe2x88x920.191, smithonite (ZnCO3) xe2x88x920.227, rhodocrocsite (MnCO3) xe2x88x920.219, and cobalt calcite (CoCO3) xe2x88x920.255. These values of birefringence are greater than those of organic compounds by at least one order of magnitude. Hence, using inorganic substances as the additive offers an added advantage in that they need be added in extremely small amounts to give the required non-birefringence.
The inorganic substance has preferably a crystal structure in the class of tetragonal, trigonal, hexagonal, orthorhombic, monoclinic and triclinic systems. In addition to these single crystal structures, polycrystals or aggregates of such single crystals may be used since those structures also cause birefringence.
The inorganic substance is not limited to minerals and ceramics can also be used. Ceramics are preferably crystalline. However, sinters comprising a multitude of crystal particles can also be used as long as they show birefringence.
Addition of the inorganic substance to the high molecular weight resin often involves the problem of poor dispersibility. To solve this problem effectively, a binder having high dispersibility in the high molecular weight resin is chosen in consideration of the inorganic substance which is then subjected to a surface treatment with the binder.
In the present invention, the inorganic substance is used as fine particles. For several reasons such as the need to develop effective birefringence in the high molecular weight resin, it is usually preferred that the particle consist of acicular, rods (cylinders) or elongated plates shaped particles. The particle size has an upper limit that is associated with the scattering of light, hence, the wavelength of light (which is to be transmitted through the optical resin material). Generally, particles not larger than about the wavelength of light are more preferred but sizes up to about several times to several tens of times greater than the wavelength of light are tolerated in practice. In particular, when the optical resin material is used as a thin film, a certain degree of scattering is seldom problematic to the transparency needed in practice and the particle size may be a little more than 100 times the wavelength of light. As an example, consider acicular particles called xe2x80x9cwhiskersxe2x80x9d that are used with visible light; the preferred thickness is not greater than a few microns, more preferably 1 xcexcm and less, and most preferably 0.5 xcexcm and less. The length is preferably not greater than several tens of microns, more preferably not greater than a few microns.
Scattering of light is closely related to the difference in refractive index between the inorganic substance and the high molecular weight resin. To be more specific, the greater the index difference between the inorganic substance and the high molecular weight resin, the higher the chance of scattering. According to the findings of the present inventor, no problems occurred in practice when the difference between the average refractive index of the inorganic substance and the refractive index of the high molecular weight resin was 0.5 or less, and the more preferred condition was 0.3 or less. The average refractive index of the inorganic substance means the average of the refractive indices of the inorganic substance in the two directions of its anisotropy in refractive index.
Chief examples of the high molecular weight resin that are commonly used as the optical resin material include polymethyl methacrylate (refractive index, n=1.49), polycarbonate (n=1.59) and norbornene based resins such as ARTON (product of JSR Co., Ltd.; n=1.51). These high molecular weight resins have refractive indices between about 1.5 and 1.6. Calcium carbonate (CaCO3) is a typical example of inorganic substances, especially minerals, that have average refractive indices not differing from the refractive indices of these high molecular weight resins by 0.3 or less. Calcium carbonate has a great advantage in that shape-anisotropic fine particles which are particularly advantageous for the purposes of the invention can be obtained more easily than when other minerals are used. Hence, calcium carbonate is particularly preferred as the inorganic substance for the purposes of the invention.
In order for the added inorganic substance to cancel out the birefringence in orientation of the high molecular weight resin by its birefringent nature, the inorganic substance has to be oriented in such a way that the axial direction of the fine particles of which it is made becomes parallel to the linked chains in the high molecular weight resin as the latter are oriented. Orientation of the inorganic substance can typically be realized by an external molding force that causes the linked chains to be oriented in the high molecular weight resin. Such orientation occurs if the fine particles of the inorganic substance are in the form of elongated shapes such as a cylinder, a columnar, an acicular and an ellipsoid of revolution. The expression reading xe2x80x9coriented in such a way that the axial direction of the fine particles of which the inorganic substance is made becomes parallel to the linked chains in the high molecular weight resin as the latter are orientedxe2x80x9d does not necessarily mean that all of the fine particles in the inorganic substance are oriented such that their axial direction is parallel to the linked chains in the high-molecular weight resin and it suffices that a statistically significant number of the fine particles in the inorganic substance are oriented such that their axial direction is parallel to the linked chains in the high molecular weight resin. The fine particles in any inorganic substance will do as long as their axial direction becomes oriented as the linked chains in the high molecular weight resin are oriented; preferably, the aspect ratio of the fine particles, or the ratio between the length in the axial direction and the diameter in a direction perpendicular to it is at least 1.5, more preferably at least 2, most preferably at least 3.
To produce the above-described optical resin material according to the invention, the inorganic substance may be mixed either prior to the start of polymerization reaction for synthesizing the transparent high molecular weight resin or after the start of the polymerization reaction but before it ends. To be more specific, the monomer as the source of the high molecular weight resin is mixed with the additive inorganic substance; after the inorganic substance is thoroughly dispersed, polymerization reaction is allowed to proceed until the optical resin material is obtained. In this polymerization process, the inorganic substance does not participate in the reaction for polymerization of the monomer for the same reason that the low molecular weight substance used in the above-described prior art does not.
There is another method that can be used to produce the optical resin material according to the invention; the high molecular weight resin material is heated to melt and the inorganic substance is added to the melt, followed by kneading the mixture to disperse the inorganic substance in the matrix. The material thoroughly kneaded by this method is preferably pelletized by a suitable means in preparation for processing into the final product. The pellets of the kneaded product are injection, extrusion or otherwise molded by ordinary techniques into the desired shape.
The main thrust of this method is that the high molecular weight resin material which has been heated to a molten state is mixed with the inorganic substance that will cancel out the birefringence in orientation of the resin material. As long as the kneading step is included, the obtained optical resin material will exhibit high degree of non-birefringence irrespective of the preceding or subsequent molding method that is employed.
The above-described method in which the inorganic substance that cancels out the birefringence in orientation is added to the molten high molecular weight resin material and the mixture is kneaded may be replaced by a process in which the high molecular weight resin material is dissolved in a suitable solvent and the inorganic substance that cancels out the birefringence in orientation is added to the solution which is uniformly kneaded and subsequently deprived of the solvent by evaporation or other method. The composition thus obtained is injection or extrusion molded into the desired shape, thereby producing the non-birefringent optical resin material.
Having the various characteristics described above, the optical resin material according to the invention can be utilized in various optical components, as well as in equipment that needs them. The optical resin material according to the invention finds particular utility as members of liquid-crystal devices. A typical example is the substrate of a liquid-crystal device placed between the liquid-crystal layer and the polarizing plate. If the substrate is formed of the optical resin material according to the invention, the aforementioned advantages of the optical resin material over the glass-based optical material can be effectively used to improve the various performance parameters of the liquid-crystal device.
The polarizing plate of the liquid-crystal device is formed by joining a transparent resin sheet to both sides of the polarizer. Applying the optical resin material of the invention to the transparent resin sheet is another advantageous method of use and the various performance parameters of the liquid-crystal device can be improved as in the case just described above.
The optical resin material according to the invention may also be used as an adhesive for permanent fixing to bond various elements of the liquid-crystal device and this is another preferred method for effectively using the high degree of non-birefringence of the resin material and the high degree of its freedom in the choice of suitable materials. With the conventional liquid-crystal device, there have been no resin materials adapted to use adhesives for permanent fixing having high degree of non-birefringence; therefore, excepting cases such as a monochromatic type where very high degree of birefringence is not required, various elements of the liquid-crystal device have been joined together by adhesives for removable fixing. If such adhesives for removable fixing are replaced by adhesives for permanent fixing that make use of the optical resin material according to the invention, the performance of the liquid-crystal device can be improved in various aspects such as endurance and heat resistance.